The consequences of insulin resistance (IR) include not only type 2 diabetes mellitus but also a cluster of metabolic abnormalities that double the risk of developing life-threatening complications of atherosclerosis including myocardial infarction, ischemic strokes, and peripheral arterial disease. The prevalence IR is increasing at an alarming rate as western populations become heavier and more sedentary. When one further considers the ongoing epidemiological transitions in developing countries in addition to the obesity epidemic in developed countries, the worldwide public health impact of IR is undoubtedly profound. Few pharmacological options exist that improve one?s insulin sensitivity and decrease the risk of complications from IR and recent genomic studies of surrogate measures of IR have yielded a disappointing number of new leads. Furthermore, a critical need exists for the development of more accurate blood-based diagnostic tests for IR. The long-term objective of the proposed research is to discover and validate novel protein markers of IR circulating in the blood of individuals who have undergone either one of the two ?gold standard? direct measures of insulin sensitivity: an insulin suppression test (IST) or a euglycemic clamp (EC). This information will be used to identify novel molecular pathways of IR that can be targeted pharmacologically and to develop statistical models that correlate highly with the degree of IR as estimated by direct measures of insulin sensitivity. In aim 1 of this proposal, the blood of 2100 white/European subjects who have undergone an IST at Stanford or an EC in the Relationship between Insulin Sensitivity and Cardiovascular Disease (RISC) and the Uppsala Longitudinal Study of Adult Men (ULSAM) studies will be measured for the presence of 981 proteins using an emerging platform that leverages novel technology referred to as the proximity extension assay. This technology allows for the accurate and reliable quantification of proteins in plasma down to the femtomolar or attomolar level. We will further validate the top signals identified in these subjects in an additional ~300 non- European subjects and a subset of 300 subjects from Stanford who underwent a second IST after weight loss or use of a thiazolidinedione. In aim 2, we will examine validated signals from Aim 1 for causality using the principal of Mendelian randomization, and we will quantify improvements afforded by validated markers over conventional measures in identifying subjects at risk of complications from IR. In aim 3, validated associations between proteins that appear causal in nature will be further examined through knockdown of the genes producing these proteins in human cell lines relevant to IR. These cell lines will include adipocytes, hepatocytes, and skeletal myocytes. This study is the largest study of the plasma proteome in relation to direct measures of insulin sensitivity ever proposed. Findings are expected to yield important mechanistic insights into the molecular basis of IR and provide the foundation for the development of a blood-based diagnostic test that can very reliably detect subjects at low or high risk of complications from IR.